nixos/acme: do not eat Let's Encrypt's request limits if misconfigured on first try (#266155)

nixos/modules/security/acme/default.nix

···
       serviceConfig = commonServiceConfig // {
         Group = data.group;
 
+        # Let's Encrypt Failed Validation Limit allows 5 retries per hour, per account, hostname and hour.
+        # This avoids eating them all up if something is misconfigured upon the first try.
+        RestartSec = 15 * 60;
+
         # Keep in mind that these directories will be deleted if the user runs
         # systemctl clean --what=state
         # acme/.lego/${cert} is listed for this reason.